It's obviously true because when you stat looking at, say, Jabber, it turns out that it only offers that for a single mobile client on Android that should connect to properly configured server with all the relevant XEPs enabled.
So yes. For practical purposes Jabber doesn't cut it. No idea about Matrix.
They have the habit to use a web service for everything, or at most a phone native "app" that they expect to be distributed by and integrated with some centralized service that exercises total control.
I've also encountered an IRC server that caches and rebroadcasts the last hour of messages.
But yeah with IRC you run into the problem that it wasn't originally designed for that use.
> Mastodon proved that you can actually build a usable app on a federated service
I don't see how anything needs to be "proved." Do we actually need proof that IRC could cache messages and repeat them for joining clients? It just wasn't originally designed that way.
XMPP also has an extension for caching the scroll.
I know i'm not the only one like that either.
People are creatures of habit, and mass products are built with either heavy marketing or piggybacked on other systems' popularity. It's hard to break into a market, but if it's any consolation it's equally hard for non-free products.